riscv: disable SUM in the exception handler

Web:        https://git.kernel.org/torvalds/c/fe9b842f72921fb18b93cf47a255f374289ef242
Commit:     fe9b842f72921fb18b93cf47a255f374289ef242
Parent:     509009ccfa53d031be5f5551fa2b7f9a4cbd2d93
Refname:    refs/heads/master
Author:     Christoph Hellwig <[email protected]>
AuthorDate: Thu Jan 4 19:55:55 2018 +0100
Committer:  Palmer Dabbelt <[email protected]>
CommitDate: Tue Jan 30 19:12:38 2018 -0800

    riscv: disable SUM in the exception handler
    
    The SUM bit is enabled at the beginning of the copy_{to,from}_user and
    {get,put}_user routines, and cleared before they return.  But these user
    copy helper can be interrupted by exceptions, in which case the SUM bit
    will remain set, which leads to elevated privileges for the code running
    in exception context, as that can now access userspace address space
    unconditionally.  This frequently happens when the user copy routines
    access freshly allocated user memory that hasn't been faulted in, and a
    pagefault needs to be taken before the user copy routines can continue.
    
    Fix this by unconditionally clearing SUM when the exception handler is
    called - the restore code will automatically restore it based on the
    saved value.
    
    Signed-off-by: Christoph Hellwig <[email protected]>
    Signed-off-by: Palmer Dabbelt <[email protected]>
---
 arch/riscv/kernel/entry.S | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 7404ec222406..87fc045be51f 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -78,10 +78,13 @@ _save_context:
 	REG_S x31, PT_T6(sp)
 
 	/*
-	 * Disable FPU to detect illegal usage of
-	 * floating point in kernel space
+	 * Disable user-mode memory access as it should only be set in the
+	 * actual user copy routines.
+	 *
+	 * Disable the FPU to detect illegal usage of floating point in kernel
+	 * space.
 	 */
-	li t0, SR_FS
+	li t0, SR_SUM | SR_FS
 
 	REG_L s0, TASK_TI_USER_SP(tp)
 	csrrc s1, sstatus, t0
--
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